Technical Field
The present disclosure generally relates to an electronic ignition system for an internal combustion engine, such as for example the engine of a motor vehicle.
More in particular, the present disclosure relates to an electronic ignition system performing the reading of the ionization current in order to measure parameters representative of the combustion process of the mixture air-fuel internally of a cylinder of the engine.
Description of the Related Art
Modern internal combustion engines for motor vehicles are equipped with systems for analyzing the internal combustion process, in order to maximize the efficiency and the performance of the engine itself.
It is known the measurement of the ionization current for obtaining information indicative of parameters of the combustion process of the mixture air-fuel directly from the combustion chamber.
In particular, the spark plug is used as an ion sensor (typically of type CHO+, H3O+, C3H3+, NO2+) which are generated in the combustion chamber after the spark between the electrodes of the spark plug has been generated and the combustion of the mixture air-fuel has taken place.
Therefore the ionization current is generated by applying a potential difference to the electrodes of the spark plug and by measuring the current generated by means of the ions produced in the combustion chamber.
By means of the measurement of the ionization current it is possible to detect the presence of oscillations of the pressure value internally of the combustion chamber (known as “knocking” vibrations), which can damage the engine head: therefore it is necessary to detect said oscillations in real time and perform timely suitable actions for preventing engine damage.
The reading path of the ionization current has a high value of impedance due to the presence of the inductance of the secondary winding which has a very high value: this makes the reading of the value of the ionization current difficult, because its amplitude value is very small.
US patent publication no. 2002/0050823-A1 discloses an ignition system having a device for measuring the ionization current.
The ignition system comprises a switch (see S1 in FIG. 1) which has the function to short-circuit each other the two terminals of the primary winding L1, during the time length of the measurement of the ionization current.
The Applicant has observed that this prior art has the following disadvantages:                the diode of MOSFET S1 enters into conduction during the operation of the coil 1, thus preventing the correct operation thereof;        the time taken for setting to zero the value of the current through the secondary winding can be too high, thus causing a delay in detecting the “knocking” vibrations;        if a plurality of spark plugs are present (typically there are four), it is required a switch for each coil connected to the respective spark plug.        